Of the naturally occurring isotopes of potassium, 40K is radioactive and decays into 40Ar at a precisely known rate, so that the ratio of 40K to 40Ar in minerals is always proportional to the time elapsed since the mineral formed [ 40K is a potassium atom with an atomic mass of 40 units; 40Ar is an argon atom with an atomic mass of 40 units].This relationship is useful to geochronologists, because quite a few minerals in the Earth’s crust contain measurable quantities of potassium (e.g. In theory, therefore, we can estimate the age of the mineral simply by measuring the relative abundances of each isotope.It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40).
Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials.Geologists have used this method to date rocks as much as 4 billion years old.Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.Radioactive elements were incorporated into the Earth when the Solar System formed.
All rocks and minerals contain tiny amounts of these radioactive elements.
Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks.
This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things.
Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off.
It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.
of grains between top and bottom is always proportional to the time elapsed.